Nature-Based Solutions for Sustainable Urban System Transformation: Addressing Circularity in Building System Recovery

Abstract

Urban system transformation in view of sustainability is fundamental for efficient adaptation and mitigation of challenges faced by cities. Sustainable urban transitions, under the umbrella of circular economy, are key to effectively addressing future challenging scenarios and their impacts. The adoption of nature-based solutions (NBS) for circular resource management can provide beneficial ecosystem services to the urban built environment while promoting the conservation and reuse of resources within the urban cycle. The Circular City framework outlined the use of NBS to tackle challenges related to urban circularity. One such challenge is ‘Building system recovery’, which involves the regeneration of the built environment. By implementing NBS, the lifespan of building systems, construction materials, buildings, as well as open spaces can be extended. This is achieved by reducing exposure to weathering from external agents, thereby reducing the rate of infrastructure renovations, retrofitting and replacements. Moreover, strategies that prioritize resource savings, greener environments, and water-sensitive systems can increase resilience by providing critical ecosystem functions such as stormwater management, greywater treatment and mitigation of the urban heat island effect. Building upon the Circular City framework, this contribution presents NBS units and interventions at different urban scales – materials, components, systems – aiming at addressing the circularity challenge of ‘Building system recovery’. This is followed by a comprehensive analysis of input and output resource streams for strengthening circularity solutions in cities. This contribution describes state-of-the-art circularity frameworks aiming at supporting decision-makers and practitioners, while providing guidance tools for involving all relevant stakeholders, thereby supporting multifunctional implementation of NBS for inclusive and resilient circular cities.

1 Introduction

1.1 Enhancing Urban Resilience by Circular Nature-Based Systems: The Circular City Framework

In recent decades, rapid urbanization and population growth have intensified environmental challenges, including climate change and ecosystem decline. The global goal is to shift towards sustainable, renewable products, and green technologies; being promoted initiatives to emphasize regenerative circular economies focusing on resource optimization, energy efficiency, and waste management. Nature-based solutions (NBS) involve integrating nature into urban environments, incorporating nature-inspired ideas into urban design. The relevance of NBS is closely tied to the concept of circular economy (CE), which advocates for restorative design solutions to minimize resource input, energy consumption, and emissions.

As an illustration of the synergies among the concepts of CE and NBS, the Circular City research framework seamlessly integrates the application of NBS in cities with the principles of CE [1,2,3,4]. This framework draws inspiration from the seven ‘Urban Circularity Challenges’ (UCCs, UCC₁–UCC₇) proposed by Atanasova et al. [5], which NBS can address effectively [6,7,8,9,10]. The UCCs that NBS can adopt within circular systems encompass: ‘Restoring and maintaining the water cycle’ (UCC₁); ‘Water and waste treatment, recovery and reuse’ (UCC₂) [8, 11,12,13]; ‘Nutrient recovery and reuse’ (UCC₃) [14, 15]; ‘Material recovery and reuse’ (UCC₄); ‘Food and biomass production’ (UCC₅) [7, 16]; ‘Energy efficiency and recovery’ (UCC₆); and, ‘Building system recovery’ (UCC₇) [1,2,3,4,5, 17, 18]. The framework englobes 39 NBS units (NBS_u), 12 interventions (NBS_i), and 10 supporting units (S_u), classified in nine sectoral categories: (1) ‘Rainwater Management’; (2) ‘Vertical Greening Systems and Green Roofs’; (3) ‘Remediation, Treatment, and Recovery’; (4) ‘(River) Restoration’; (5) ‘Soil and Water Bioengineering’; (6) ‘(Public) Green Space’; and, (7) ‘Food and Biomass Production’. A detailed methodology on the inputs and outputs (I/O) resource streams from the NBS_u/i is provided within the framework [1, 2, 7, 8]. To enhance the accessibility and usability of the framework for interested stakeholder groups, a comprehensive graphic tool has been developed. The guidance tool facilitates the automatic quantification of NBS_u/i resource streams – encompassing both I/O, and includes a descriptive toolbox that offers useful knowledge and guidance for users [19, 20].

The Building System Recovery Challenge.

The seventh UCC (UCC₇) addresses the pivotal theme of regenerating the built environment; embracing architecture and infrastructure tailored for living, working, manufacturing, and various other activities. Through effective shielding from UV (ultraviolet) radiation and pollutants, buildings and open spaces contribute to the prolonged lifespan of prevalent building materials, consequently diminishing the frequency of necessary renovations or infrastructure replacements. This proactive approach not only extends the longevity of structures but also yields resource conservation benefits [1, 4, 5].

2 Materials and Methods

2.1 Holistic Approach to the Urban Built Environment

The COST Action CA21103 “Implementation of Circular Economy in the Built Environment” (CircularB) constitutes an excellent platform for science communication and knowledge sharing among a variety of disciplines and sectors. In the present research study, contributed CircularB experts from the fields of Engineering, Architecture, Urban Planning, and Environmental Sciences. Thus, transnational research opportunities served as a valuable tool to enrich the UCC₇ perspectives on the potential of NBS implementation by addressing CE and sustainable design strategies in the urban built environment.

Selection and Implementation of Nature-Based Solutions to Address the Building System Recovery Challenge.

This research stands from the selection of NBS_u/i and S_u which contributes to the ‘Building system recovery’ challenge [1]. Considering relevant NBS_u/i and S_u selected by Langergraber et al. [1], CircularB experts complemented the framework of NBS relevant for the UCC₇; and, proposed an approach on how this is related with the UCCs network. Additionally, a case study is analyzed and presented as an exemplary best practice addressing the UCCs; and specifically, the UCC₇ [1, 2, 4, 5].

Fórum da Maia Case Study Implementation.

The green roof system at the Maia Forum, Maia, Portugal was executed as part of BaZe (Maia Net Zero Carbon City, Living Lab); a living laboratory dedicated to decarbonization. One of the primary project objectives was to showcase a building solution inspired by nature; illustrating in practical terms, the manifold environmental, financial, and social benefits.

Description of Demands, Services and Quantification of Resource Streams related to Building System Recovery.

This approach would enrich the Circular City guidance tool, by proposing the integration of the UCC₇, and, consequently the UCC₄ on ‘Material recovery and reuse’ [19, 20]. This, it will also serve as a basis for the development of the common international framework from the COST Action CircularB on circularity indicators.

3 Results and Discussion

3.1 Nature-Based Solutions of Relevance to Address the Building System Recovery, and Proposal of New Units to the Circular City Framework

Identification of Nature-Based Solutions to Potentially Address the Building System Recovery.

This study complements the selection of NBS_u/i and S_u proposed by the Circular City framework [1]. Thus, different levels of implementation were proposed (Table 1), as well as the inclusion of new NBS_u/i into the framework which potentially addresses the UCC₇.

Table 1. The urban circularity challenge (UCC) on ‘Building System Recovery’ (UCC₇) addressed by nature-based solutions (NBS) units (NBS_u), NBS interventions (NBS_i), and Supporting units (S_u) (based on and adapted from Langergraber et al. [1] and Canet-Martí et al. [7]).

Fig. 1.

A schematic simplified representation of the Urban Circularity Challenges (UCCs) integrated at the urban built environment – Building scale exemplification of the seventh UCC, on ‘Building system recovery’, and its associated circularity challenges network.

Potential NBS_u/i addressing the UCC₇, their descriptions and synonyms, are suggested to the aim of enhance the Circular City framework – as they were not included originally. This would be the case of bio-solar green roofs (Fig. 1), that would be characterized within the ‘Vertical Greening Systems and Green Roofs’ category (Table 1) [1,2,3] as integrated solar renewable energy (photovoltaic, PV) systems in green roofs; thus, improving biodiversity and energy efficiency.

Associations Among Building System Recovery and Other Urban Circularity Challenges.

Urban greening and water aspects, and specifically related with vegetation in cities, are among the essential elements for the successful implementation of NBS at the urban scale – e.g., both as an input and output resource stream (UCC₅, biomass production) [1, 2, 4, 7]. Designing urban water integrated NBS_u/i, implying systems with diverse design and purposes, aimed at addressing the UCC₇, also implies the consideration of other challenges, such as the UCC₁, UCC₂ and UCC₃ by adopting critical water resource strategies – e.g., the use of greywater flows for irrigation, and other purposes such as non-potable water; specially, in those drought sensitive areas (Fig. 1). Urban planning instruments, policies and strategies benefiting from inter- disciplinary and sectoral collaborative networks are essential to boost the implementation of NBS in cities; being addressing multiple UCCs, simultaneously (Fig. 1) [2, 4, 7].

Fig. 2.

Proposed role of ‘Building system recovery’ challenge in the Circular City toolbox (‘Descriptive Toolbox’) [3, 20] to promote the information model based on potential urban resource flows related to the seventh Urban Circularity Challenge.

Potential Integration of Building System Recovery Challenge on the Circular City Toolbox and Guidance Tool.

The graphic tool developed within the Circular City Toolbox includes different UCCs levels, such us: ‘Water and Waste Treatment and Recovery’; ‘Restoring and Maintaining the Water Cycle’; ‘Food and Biomass Production’; ‘Nutrient Recovery and Reuse’; and, ‘Energy Efficiency and Recovery’ (Circular City website, guidance tool [19, 20]. This study proposes the inclusion of the UCC₇ – to be complemented by the UCC₄, as part of the tool levels (Fig. 2). Thus, new demands – such as ‘Alternative component source’, ‘Building reuse’, and ‘Regeneration’; and services – ‘Collection/separation’, ‘Improve building insulation’, and ‘Microclimate regulation’ were included (Fig. 2). The challenge of ‘Building system recovery’, understood under an integrative circular thinking design, can be addressed by NBS_u/i (Table 1) [1] working as interconnected individual units or interventions where the resources flow by closing a common urban circular system (Fig. 2) [1,2,3, 19, 20]. Thus, the process of urban planning would been enriched from sectoral perspective integration aimed at giving value to the multifunctionality of NBS systems.

3.2 Building System Recovery Best Practices: The Case of Fórum Maia

Design concept for the green roof at Maia Fórum (Fig. 3) was developed concurrently with spatial analysis; considering building characteristics, and the educational aspect intended from the project. Thus, a deliberate choice was made for a straightforward and naturalistic design focused on enhancing biodiversity, maximizing greenery (addressing UCC₅), and facilitating occasional visitor access in a non-disruptive manner. Design encompassed terrain modeling and vegetation composition, predominantly using native species – e.g., Verbena bonariensis, Thymus serpyllum, and Corynephorus canescens; highly adapted to regional climate, and well known for their effectiveness in promoting biodiversity. Two main vegetation layers vary between lower-growing species, about 20 cm; and, intermediate species, about 1 m in height. Their diverse textures, growth patterns, and flowering cycles contribute to a visually pleasing composition that maintains sensory appeal throughout the year. This green roof is intended to function as a living laboratory, for which thermal and humidity sensors have been installed, along with an interconnected and monitored meteorological station (monitoring UCCs_1,2,6). The project comprises the use of an ecologically designed green roof system, entirely produced in Portugal, based on expanded cork agglomerates (black cork blocks) (accomplishing UCCs_4,7). Among the main ecosystem services it provides, the following are identified: (i) promotion of flora and fauna biodiversity within the urban built environment (UCCs_5,7); (ii) retention and reduction of the rainwater drainage rate, and promotion of sustainability of urban drainage systems (UCCs_1,2,3,7); (iii) comfort and thermal efficacy, providing better insulation and contributing to enhance the energy efficiency of the building (UCCs_6,7); and, (iv) mitigation of the urban heat island effect by absorbing and dissipating heat, thus reducing the overall temperature in urban areas (UCCs_6,7). This contributes to a more comfortable and balanced microclimate and helps combat the higher temperatures typically experienced in densely built urban areas.

Fig. 3.

Semi-intensive green roof implemented at Fórum da Maia, Maia, Portugal.

4 Conclusions

Cities must transform for sustainable futures; thus, reinforcing their ability against existing urgent challenges, such us resource scarcity, climate change, and ecosystem degradation couple with biodiversity loss. Nature-based solutions (NBS) as multifunctional systems, offer valuable ecosystem services to the urban biosphere. Moreover, by adopting the concept of circular economy, NBS implementation supplements the support towards sustainable transformation of the urban built environment. NBS in urban regeneration effectively addresses the ‘Building system recovery’ challenge (UCC₇), with circular buildings positively impacting materials, energy, waste, health and well-being, and biodiversity (UCCs_1–6).